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Abnormal Agonist Syndrome, commonly known as "demonic possession" prevailed in Japan. Patients of the syndrome have malfunction in the secretion of the neurotransmitters and it gives rise to the acquisition of new parts in the body and supernatural power. The patients of "A-Syndrome" were unnaturally concentrated in Shikura city.

DDD has no current stated connections to the rest of the interconnected worlds of the Nasuverse. Although Meruka Kuramitsu from Kara no Kyoukai is referenced in the story, it is simply a gimmick included by Nasu to cause readers of both to do a double-take while reading.[1] It is actually a pseudonym taken from a character in a book in the DDD world.

Arika Ishizue lost his left arm in the A-syndrome related accident and hospitalized. After he left the hospital and went back to Shikura, he met a beautiful boy named Kaie Karyou who has no limbs and is living in a dark basement. The boy asked him to exorcise the "Demons" possessing A-Syndrome patients. Arika was involved in the bizarre cases and the fights with the "Demons".

Shikura City is the DDD setting. Which is located in western Japan with an population of 150,000 people. More than half of town is made up of fields and mountains. No matter how much money gets invested in the station front, it’s still a town out in the country, two hours away by commuter express from the urban center.

Water Tank - Unmarked on the map and located in a forest five kilometers away from the station of Shikura. Looks sort of like a spaceship. Underneath it is Kaie's room.

SVS - A popular game among young people who couldn't become serious Baseball players. Only 2 players are needed- a pitcher and a batter. The pitcher pitches the ball and it's simulated on whether it would be an easy out or whether it's a good hit or not.

Origa Memorial Institute - A national hospital and the one and the ultimate treatment center for those with Agonist Disorder. Those with "Agonist Disorder" are transported to this hospital after being taken into custody by the government. Blocked off by a concrete wall, even at the 10 meter tall glass entrance of the hospital. Incoming and outgoing patients come and go by helicopter. Said to be located in western Japan and is some 3 hours away from Shikura by helicopter/car. Doesn't take outpatients; if you come there, you're going to be staying there for a while. In that sense, it's more like an asylum. Made up of a large compound and 5 wards (A, B, C, D specifically for those with A Disorder) and relatively small number of staff in comparison to the size (only in the hundreds). Patients in each ward are allowed to move only to the adjacent wards and only according to allotted free time.

In WardA are those with tumors and no new parts (abilities) or those normal people who just happened to be harmed by those with Agonist Disorder and bear some after-effects. Basically the same as regular hospital life, excluding a limit to free time and lack of outcoming patients. Three examinations a day and optional communication with other patients. Around some 20 A ward patients.

WardB has the most number of patients. Mainly those who have become comparatively mentally stabilized and whose treatment is just barely realistically possible. Basically the people who probably will be allowed to get surgery. Treatment is basically at a level where excision of tumor = won't die. Whether surgery is commenced with depends on whether an accurate surgical method is discovered. A problem however is that the symptoms are different so research into that field, to develop a generally effective practice, is slow.

Ward C are where those who are popularly called "demon possessed" reside, of course, their conditions have stabilized. Oddly enough, they're less dangerous than those in Ward B since these ones have been emotionally shattered and will not leave their wards. Those allowed of free action have already stabilized so they won't go violent; but the result is that it looks like a prison-like abandoned ward since patients whose bodies have mutated won't leaving their rooms, even if they are emotionally peaceful.

Ward D can best be described as being abandoned, if Ward C were to be described to be a prison. Security (equipped openly with submachine guns) and doctors can only be found in the entrance. Left dark since many of the patients are fearful of light. Basically made up of those patients in the terminal stage whose bodies have matured and treatment is impossible. Around 30 people are alive in there and about 40 bodies are "currently in" the room. A steel corridor maze complete with layers and layers of steel gates.

Tsuranui is Arika's slightly eccentric kouhai. She has a crush on him An old tie back from high school that Arika can’t get rid of, she’s always been like this since way back. To sum her up roughly, she’s cheerfully lively and a straightforward fumbler that can’t lie. Arika has problems dealing with her as she’s a monster that’s jump from statement to execution in no more than a few seconds. She is currently living alone in an apartment near the university on the edge of the city's industrial area.

Mato Touma (戸馬 的, Touma Mato?), or more affectionately known as Tomato.

She is a female cop, in her twenties, that is very interested in demon-possession but not to the lengths of becoming one of them herself. After all, she’s a Public Security special agent. You’d have to be a superintendent to voice your opinions to her. As a matter of fact, in Shikura City, she’s invincible. Oh, and just to let you know, a superintendent would be the chief of police in Shikura City. She is a cold-blooded animal that Arika had been under complete supervision of for two years but she apparently has a nice side to her as well. The true nature of that woman is a sadist that loves tormenting weaklings like Arika and in the eyes of Arika, isn’t a shred of justice in this woman. But according to Mato, is Akira a harmless demon possessed and gets used by her to get information about demon possessed one's. She was the one that introduced Arika to Kaie and his limbs. She is, also a weapons expert and deadly with shotguns and pistols. She also, contrary to her nickname, hates tomatoes.

Kirisu is Arika and Tsuranui's friend and an ex-yakuza gang member who helps Arika by finding AD. He assisted in beating the Sinker and is one of the major characters in SVS. He was friends with Iguruma when they were in middle school. He left the yakuza after finding his friend's mom being used as a laborer for them. So he beat the guy who was using her (with a bat) to death and now cannot hold a Baseball bat without thinking about it. So he gave up sports. And the guy he beat to death, wasn't really liked at all so he could come back but he didn't really want to stay anymore.

A friend of Kirisu in middle school who is a star baseball player who fell from grace when his teammates break his pitching arm when they get too envious of his skills. His mom eventually dies by suicide after finding out he tried to continue pitching despite his broken elbow. He was eventually expelled from school and made a homeless wanderer. After a few months, a mysterious man bestowed upon him his powers in exchange for his life and sanity. After contracting A-disorder, he goes around killing those on the street playing a game called SVS, a simplified version of baseball. After he drove himself insane, he embarked on a maddened slaughter spree. It didn’t matter who died, as long as they died. His new function is located in his pitching arm and it allows him to throw something with the ability to turn it in completely different directions in midair and the thrown object is totally invisible because of the spin used when throwing. And to use those powers he has to cover these objects with his own blood. However, when he over uses his powers and runs low on blood he has a decreased perception, reduced ability to think and his body temperature drops low.

Hinomori is a hunter of the demon-possessed and at the same time known as the “immortal vampire”. His reasons for going around killing Agonists is unidentified and he walks around with a woman he calls 'Maki-chan'. Being a demon possessed himself his power is basically immensely heightened reflexes cause by a condition his brain has that allows him to perceive the world in a adrenaline-enhanced state, similar to that of a human being when they know their life is in danger and death is imminent. He fights with a half-cut meter long pipe which was sharpened into a stake. He has shark teeth, wears a long black coat all the year and has sunglasses. He fights Tomato on one occasion and demonstrates speed enough to dodge bullets from a H&K MP5 SMG. During the case with the sinker, he gets acquainted with Arika and later recognizes him as a friend. He himself takes the law very lightly and is outrageously stupid by nature. But also at the same time he is very brutal and ruthless as he is also a wanted serial killer.

Makina is Shinya’s older sister who has the ability to freely change her muscle fibers and the nerves of her face. She can change her appearance at her own will and she can even copy thought patterns and weird habits. In her case, she tried to copy her brother because of apparent suffering at the hands of her parents. She exists as a hollow person who could not exist without wearing the face of another. She assumes her brother’s identity and becomes an exact carbon copy of Shinya. Makina's imitation of Shinya began to take over the original by becoming a better Shinya than even Shinya himself. Shinya worked hard to gain the popularity and accomplishments he enjoyed, but Makina's Shinya did even better simply by imitating. Despite the irony in that, those who once supported Shinya soon switched camp to the new Shinya. She thought of an psychotic master plan engineered to drive him insane and kill her parents. After this is done, she assumes Arika’s identity as soon as Shinya’s parents are killed in a freak accident. Shinya, however, manages to smoke her out by committing a series of crimes as Arika culminating in his eventual death. With no facade left to wear, she tries to copy Kaie as a final, last act of vengeance. She dies when trying to assume Kaie’s existence, only to find out she cannot.

Yamanashi had killed her parents and then in order to hide the motive, begun she to kill other humans. She killed her parents because they let her bedridden grandfather, who was a burden for their family, die without helping him. Tomori was as a witness of her grandfather's death and she couldn't forgive her parents or the villagers for not doing something. She has long black hair, luminous eyes and a lovely voice. But the Agonist Disorder had transformed her into a human beast with claws as hands and she is now known as the "lawn mower". Her claws are disintegration weapons, who will tear of all things who are caught in them. Combined with her acrobatic, speed, accuracy and power she gained from the Disorder, is she an powerful enemy. She additionally acquired an instantaneous sense of danger, thanks to her two years of human hunting experience. Hoewer, the serial killings she made, had lead the hunter of the demon-possessed, Syuusei, to her. She and Syuusei had a confrontation on a bridge and as far as physical ability goes is Yamanashi ahead of Hinomori. But she was overwhelmed by the difference of combat experience and she lost. She was later taken into custody by the Olga Memorial Hospital Ward C and they imprisoned her in a cage and her fate is unknown.

Shinya is one of the agonists admitted into a place known as the Origa Memorial Hospital, wherein Arika was also briefly warded. He was warded for A-disorder under the grounds of trying to kill his parents. He was discharged later on counts of being “rehabilitated” and that the incident was deemed to be an accident. "Shinya Hisaori", a fellow patient who befriended Arika when they met during their free activity times.

Yukio is a high school girl with intensified physical abnormalities. Because of anorexia and fasting she gained agonist disorder. She wants to find food that does not make fat due to her fasting. And so came she to the conclusion that she should eat pet animals or humans. She usually sprinkles her food with vinegar. Her disorder lies in her stomach and so she produces highly corosive stomach acid which she can vomit or sweat out of her body.

Kizaki is a man whose affected area is the neck, caused through fatigue. People who meet his eyes are bound to rotate in the same way to fit his neck, but they die when their heads turn around 180 degrees. He himself does not die as his head rotates to 360 degrees. He used this method to kill his wife and daughter and two policemen.

He is a super NEET who has a large role in Outside the Universe. He has created a concept universe within his room, which changes depending upon how it is perceived to work. The laws of the universe can be warped depending on the circumstances.